1. Field of the Invention
This invention relates generally to a catheter assembly for measuring fluid pressures in body cavities and more specifically to a fiber optic catheter assembly adapted for use with a variety of monitors.
2. Description of the Prior Art
Systems for measuring fluid pressure in body cavities have typically included liquid filled catheters which have communicated a fluid pressure inside the body to a pressure sensor outside the body. The accuracy of this system has suffered due to variations in hydrostatic pressure and other inconsistencies associated with the fluid column.
The sensors used with these systems have typically consisted of a pressure responsive diaphragm in fluid communication, via the fluid-filled catheter, with the body cavity. Pressure induced deflections of these diaphragms are mechanically coupled to piezo-resistive strain gauges which alter their resistance in accordance with well known principals. These strain gauges are usually configured in a Wheatstone bridge arrangement. The amount of induced strain, hence applied pressure, is determined by applying an excitation voltage to the bridge and then monitoring the bridge output voltage.
Typically the sensors are provided in a device separate from the monitor or display instrument, and are connected to the monitor via an electrical cable and a connector which may be disconnected for service, patient transfer, or disposal in the case of single patient use. Patient monitors on the other hand are often permanently installed within the operating room or intensive care unit of a hospital. These monitors often include inputs for other devices such as electrocardiogram leads.
With these systems of the past, a standard has been adopted wherein the patient monitor supplies an excitation voltage to the sensor, and the sensor provides an output voltage to the monitor. In accordance with the principles under which Wheatstone bridge sensors operate, the output voltage is proportional to the excitation voltage and also proportional to the applied voltage. Over time a proportionality constant has been standardized so that five microvolts of signal per volt of excitation is equivalent to one millimeter of mercury applied pressure. Using this standard, any sensor could be readily adapted for use with any patient monitor which also adhered to this standard.
The proportionality standard enabled users to realize a significant advantage in using these systems . . . many different types of sensors and patient monitors from various manufacturers could be readily interchanged. As a consequence, systems based on this standard, have achieved almost universal acceptance despite the difficulties associated with pressure measurements through a fluid filled catheter.
The adoption of this proportionality standard admits the possibility that the excitation voltage can be of almost any magnitude and may even be time varying. Furthermore, since the technology dictates the use of piezo-resistors having a certain minimum resistance, these sensors generally consume very little power. As a result, the monitors have been freed to supply excitation power having varying voltage levels and formats, both time-varying (sinusoidal and pulsed) and time independent (DC). This enabled excitation voltages to be supplied and configured in accordance with the requirements and desires of the individual monitor manufacturers.
The Wheatstone bridge circuits also have very low power requirements. As a result, the excitation power supplies of the monitors have been designed to provide only limited amounts of power.
Recently, applicant has disclosed an optical sensor and assembly that may be placed directly within the body cavity to be monitored. This eliminates many of the deficiencies associated with using external sensors. It is now desirable to adapt this optical catheter assembly so that it too can function with substantially all of the patient monitors presently available.